1. Field of the Invention
The present invention relates to a nickel metal-hydride cell. In particular, the present invention relates to a nickel metal-hydride cell having improved storage properties at a high temperature.
2. Prior Art
With the progress of the miniaturization of portable electronic equipment, safer secondary cells having larger capacities are desired. Thus, various investigations have been made to further increase the capacities of nickel metal-hydride cells comprising a hydrogen absorbing alloy as a negative electrode active material. As a hydrogen absorbing alloy used as the negative electrode active material, Laves alloys (AB2 type alloys) comprising Zr, Ni, V and Mn, Mgxe2x80x94Ni alloys (A2B type alloys) comprising Mg and Ni, and also rare earth metal alloys (AB5 type alloys) comprising a rare earth element and Ni are well known. Among them, misch metal (Mm) alloys (MmNi5) comprising a misch metal as a rare earth element is widely used as an electrode material since it can be easily activated and has a high ability to absorb and desorb hydrogen.
It is suitable for the hydrogen absorbing alloys to have a hydrogen equilibrium pressure of 20 to 500 kPa at room temperature judging from the charge-discharge efficiency, in addition to the high hydrogen absorbing capacity. However, the hydrogen equilibrium pressure of the MmNi5 type alloys is as high as 1 MPa at room temperature, and therefore it should be lowered. Furthermore, the MmNi5 type alloys have problems that the alloys are finely pulverized and suffer from the change of the composition and therefore the hydrogen absorbing ability and the reactivity tend to decrease, when they are repeatedly charged and discharged in an alkaline electrolytic solution.
Thus, MmNi5 type alloys in which a part of Ni is replaced with other metal such as Mn attract attention, since they have a low hydrogen equilibrium dissociation pressure and good hydrogen absorbing abilities, and the life of the alloys can be extended in the charge-discharge reaction. Such misch metal alloys are disclosed in JP-B-5-15774, JP-B-5-86029, JP-A-1-162741, etc., and are characterized in that they have the substantially stoichiometric composition containing a relatively large amount of cobalt to prevent the pulverization and to improve the corrosion resistance against the electrolytic solution. In addition to such alloys having the stoichiometric composition, those having a non-stoichiometric composition containing the increased amount of elements on the B site (Ni site) are proposed.
With the miniaturization of the portable electronic equipment which uses a cell as a primary powder source, the occasions to carry and use the cells increase, and the cells are used in a wider variety of circumstances than ever. Thus, it is required for cells to exert the steady performances without being influenced by the change of environment, in particular, the change of temperature. For example, when a cell is used in a lap top type personal computer or a cellular phone, it is required to exhibit the same performances at low or high temperature as those at room temperature. Therefore, the requirements for the cells become severer. Accordingly, in the case of cells used in such electronic equipment, the further improvement of temperature characteristics is being studied.
However, a nickel metal-hydride cell, which comprises the above misch metal type alloy as a hydrogen absorbing alloy used as a negative electrode active material and a paste type nickel electrode as a non-sintered type positive electrode, suffers from the severe decrease of the cell voltage, when it is maintained at a high temperature atmosphere of 80xc2x0 C. In particular, a multi-element type misch metal alloy, in which a part of Ni elements of MmNi5 are replaced with at least Mn, suffers from the significant decrease of the voltage under the high temperature atmosphere.
Since the decrease of the voltage during the storage in a high temperature atmosphere will have large influences on the cell capacity after storage, it is very important to improve the high temperature storage properties of the nickel metal-hydride cells.
One object of the present invention is to provide a nickel metal-hydride cell comprising a rare earth metal based hydrogen absorbing alloy as a negative electrode active material and a paste type nickel electrode as a non-sintered type positive electrode, which has improved storage properties at high temperature, that is, which suffers from less decrease of the voltage during the storage in a high temperature atmosphere and has a large recovery rate of a capacity after the storage in a high temperature atmosphere.
To achieve the above object, the present inventors studied the decrease of the voltage of a nickel metal-hydride cell after the storage in a high temperature atmosphere, when the cell comprises a non-sintered type positive electrode consisting of a paste type nickel electrode to which cobalt or a cobalt compound is added as a conducting aid, and a negative electrode comprising a MmNi5 type hydrogen absorbing alloy containing Mn. Thus, it was found that, as shown as Curve 1b in FIG. 2 (the results obtained in Comparative Example 1), the voltage decreased by two steps. As a result of the investigation into the causes of the voltage decrease, it was found that the first step decrease of the voltage down to about 0.9 V was due to the reduction of the positive electrode with hydrogen which was desorbed from the hydrogen absorbing alloy as the temperature rose, while the second step decrease of the voltage down to 0 V was due to the micro-short circuits formed in the cell.
Furthermore, it was found that the first step decrease of the voltage was caused as follows:
As the ambient temperature rises during the storage, the equilibrium dissociation pressure of the hydrogen absorbing alloy increases, and thus hydrogen, which is absorbed in the negative electrode as a discharge reserve and may not be essentially desorbed, is desorbed in the cell in the form of a hydrogen gas and then reduces the positive electrode. As a result, the storage properties at high temperature deteriorate. The reason for this phenomenon may be thought as follows:
The positive electrodes of the nickel metal-hydride cells include a sintered type positive electrode and a non-sintered positive electrode which is a paste type nickel electrode. The non-sintered type positive electrode may be produced by dispersing nickel hydroxide, a binder, a thickening agent, etc. in water or a solvent to form a paste, and filling the paste in a conductive porous material which functions as a collector. In this case, a distance between the active material and a substrate increases and thus the utilization ratio of the active material tends to decrease. To increase the utilization ratio to achieve the high capacity, a cobalt conducting aid such as metal cobalt, cobalt monoxide, cobalt hydroxide, etc. is added to the positive electrode. Such cobalt conducting aids form cobalt oxide, for example, CoOOH which has a higher valency than the divalent in the course of charging, and thus a conductive network of cobalt oxide oxide which electrically connects the particles of nickel hydroxide is formed.
In the nickel metal-hydride cell, the generation of gasses from the negative electrode in the final period of charging or discharging is suppressed by making the capacity of the negative electrode larger than that of the positive electrode so that the oxygen gas generated from the positive electrode can be absorbed with the negative electrode. Thus, the tight sealing of the cell is achieved. That is, even after the positive electrode is fully charged in the course of charging, or completely discharged, the negative electrode still has a non-charged or non-discharged part, which allows the positive electrode to preferentially generate oxygen gas. Therefore, the generation of the gasses from the negative electrode is suppressed. The excessive capacity of the negative electrode in the period of discharge, that is, the quantity of electricity of the discharge reserve of the negative electrode does not directly contribute to the charge-discharge reactions, but it is necessary to discharge the positive electrode capacity to the end of the discharge. Therefore, the excessive capacity is formed in the negative electrode as a counter reaction to the above-described reaction to form the conductive network of cobalt oxide in the positive electrode.
However, when such nickel metal-hydride cells are stored in a high temperature atmosphere after discharging, an amount of hydrogen, which is stored as an excessive capacity of the negative electrode on the discharge side, that is, the quantity of electricity of the discharge reserve, is desorbed from the negative electrode as the temperature rises. Then, the desorbed hydrogen gas reduces the conductive network of cobalt oxide in the positive electrode, and thus decreases the voltage of the cells.
The above phenomenon was studied. As a result, it was found that, in the paste type nickel electrode, the discharge reserve contains a quantity of electricity which exceeds an amount corresponding to the counter reaction against the oxidation reaction of the cobalt conducting aid for maintaining the conductivity of the positive electrode, and the excessive amount is further increased by that corresponding to the corrosion reaction of the hydrogen absorbing alloy which acts as the negative electrode active material.
The misch metal alloy in which a part of Ni elements are replaced with other metal can decrease the hydrogen equilibrium dissociation pressure, improve the hydrogen absorbing ability and prolongs the life of the alloy in the charge-discharge reactions. However, since such a misch metal alloy containing a replacing metal has a more active surface than the MmNi5 type alloy containing no replacing metal, the excessive quantity of electricity corresponding to the corrosion reaction of the alloy in the electrolytic solution is large, and therefore the hydrogen amount in the discharge reserve tends to increase. As a result, the voltage of the cells remarkably decreases during the storage in a high temperature atmosphere. In particular, when the misch metal alloy contains Mn as one of the alloy metals, Mn tends to be dissolved in the form of Mn ions in the electrolytic solution during the storage. Thus, the quantity of electricity corresponding to the counter reaction against the oxidation of Mn is stored as a part of the discharge reserve and thus the quantity of electricity of the discharge reserve further increases. When the Mn ions migrate to the positive electrode, they are oxidized at the positive electrode so that the conductive network of cobalt oxide is further reduced, since the electric potential of the positive electrode is higher than that of Mn ions.
Next, the second step decrease of the voltage may be caused as follows:
When the conductive network of cobalt oxide, which electrically connects the particles of nickel hydroxide, is formed, the cobalt ions formed by the dissolution are deposited on a separator which is in contact with the nickel electrode as the positive electrode, and thus the cobalt ions are oxidized to form cobalt oxide in the first charging cycle. However, at this time, the cobalt oxide is not deposited on a separator which is in contact with the hydrogen absorbing alloy electrode as the negative electrode. Therefore, no short-circuits are formed.
However, since the cells are often stored in a high temperature atmosphere after the finish of discharging, the positive electrode is reduced with the hydrogen gas desorbed from the hydrogen absorbing alloy in the first step as the temperature rises, and thus cobalt oxide with the higher oxidation state in the positive electrode may be reduced to cobalt oxide with the lower oxidation state which is soluble in the alkaline electrolytic solution. Therefore, cobalt ions are formed by the dissolution of cobalt oxide. The cobalt ions gradually migrate to the negative electrode and reduced to form metal cobalt. As a result, metal cobalt is deposited on the negative electrode during the storage of the cells at high temperature, and the deposited metal cobalt forms micro-short circuits with cobalt oxide which is deposited on the separator from the positive electrode. Thus, the voltage of the cells is decreased. That is, it is assumed that the second step decrease of the voltage is caused in correlation with the first step decrease of the voltage.
Then, based on the above findings, the present inventors have made further study to solve the above problems, and have completed the present invention.
Accordingly, the present invention provides a nickel metal-hydride cell comprising:
a paste type nickel positive electrode containing nickel hydroxide and a cobalt conducting aid selected from the group consisting of metal cobalt and cobalt compounds,
a negative electrode which comprises a hydrogen absorbing alloy having a composition of the formula:
MmNi5xe2x88x92x+yMx
wherein Mm is a rare earth element containing at least La, M is a metal element containing at least Mn, x is a number larger than 0 and smaller than 2 (0 less than x less than 2), and y is a number larger than xe2x88x920.2 and smaller than 0.6 (xe2x88x920.2 less than y less than 0.6),
a separator interposed between the positive and negative electrodes, and
an electrolytic solution comprising an aqueous alkaline solution, in which a ratio of Cxe2x80x94H to Cxe2x80x94Co(II) is 1.3 or less (Cxe2x80x94H/Cxe2x80x94Co(II)xe2x89xa61.3), wherein Cxe2x80x94H is a quantity of electricity of a discharge reserve formed in the negative electrode, and Cxe2x80x94Co(II) is a quantity of electricity necessary for reducing cobalt oxide with a higher oxidation state in the positive electrode to cobalt(II) oxide.
Herein, the quantity of electricity necessary for reducing cobalt oxide in the positive electrode to cobalt(II) oxide (Cxe2x80x94Co(II)) means a quantity of electricity which remains after the discharge of nickel hydroxide, as can be understood from the above explanation. However, since the reducing rate of cobalt is very low and no discharge occurs under usual discharging conditions, the above quantity of electricity Cxe2x80x94Co(II) means a quantity of electricity when the cell after discharging is further discharged at a microcurrent to 0 (zero) V.
Herein, the quantity of electricity of the discharge reserve formed in the negative electrode (Cxe2x80x94H) is obtained by measuring the hydrogen gas contained in the negative electrode of the cell after discharging by a water-replacing method and converting the hydrogen volume to a mole amount.